Progesterone (P4) is essential to maintenance of primate gestation and cortisol (F) to fetal lung maturation and possibly the onset of labor. The regulator factors involved in production of these steroid hormones, however, particularly during mid-late gestation are unknown. Over the past 8 years the principal investigators have been elucidating the patterns of maternal and fetal endocrine production during normal baboon pregnancy. From their studies these investigators have demonstrated the baboon to be an excellent model for the study of human pregnancy. Continuing studies by these investigators are also elucidation specific factors in the regulation of maternal-fetal steroid production. The current studies will determine in pregnant baboons the role, site and mechanism of action of estrogen and influence of the fetus upon P4 production and the influence of estrogen, the fetus and advancing gestation upon transuterine corticosteroid metabolism. The study should yield new insight into the regulation and interaction between estrogen, progesterone and corticosteroids within the maternal, placental and fetal compartments at mid to late baboon gestation.